Usually to evaporate a pure liquid, said liquid is made to flow, together with the vapour which forms in the neighbourhood of the heat exchange surface, in the same upward direction, with the liquid initially filling the entire cross-section of the inside of the tube on its side of the heat exchange surface.
Such a method is entirely suitable when the liquid to be evaporated is available in large volumes which can be discharged at a high rate, in particular in the case where water is to be evaporated.
When only a limited volume of liquid to be evaporated is available, e.g. when said liquid is relatively expensive and constitutes an intermediate heat exchange fluid between two other fluids, such a method ties up a large volume of liquid circulating in a closed circuit.
Preferred implementations of the present invention remedy said drawback by providing a method of evaporating a pure liquid which method allows optimum use of the heat exchange capacity of a heat exchange surface for a relatively small volume of liquid to be evaporated. In particular, the invention can be used to evaporate or heat a liquid flowing at high speed using heat from a fluid with an appreciably higher boiling point by means of a small volume of an intermediate heat exchange fluid circulating in a closed circuit.